Helical coil heat exchanger

ABSTRACT

A heat exchanger, particularly applicable for transferring heat from a reactor-heated primary coolant circuit to a secondary coolant circuit, wherein the secondary coolant flows upward inside helical tubes in a counterflow arrangement with respect to the primary coolant, and the ends of the helical tubes are attached to tube sheets which are accessible from above for inspection and repair, and a method for locating and repairing leaking conditions. The invention is applicable, for example, as an intermediate heat exchanger in a sodium to sodium system for large liquid metal fast breeder reactors.

United States Patent Boardman et al.

[451 Apr. 23, 1974 HELICAL COIL HEAT EXCHANGER Inventors: Charles E. Boardman; John H.

Germer, both of San Jose, Calif.

The United States of America as represented by the United States Atomic Energy Commission, Washington, D.C.

Filed: Dec. 12, 1972 Appl. No.: 314,482

Assignee:

US. Cl. 165/163, 122/32 Int. Cl. F28d 7/02 Field of Search 122/32-34;

References Cited UNITED STATES PATENTS 3/1964 Bonivi et al 122/32 X 4/1966 Ammon et a1. 122/32 X 3,338,301 8/1967 Romanos 122/32 X Primary Examiner-Charles J. Myhre Assistant Examiner-Theophil W. Streule, Jr. Attorney, Agent, or Firm-John A. Horan; F. A. Robertson; L. E. Carnahan [5 7] ABSTRACT A heat exchanger, particularly applicable for transferring heat from a reactorheated primary coolant circuit to a secondary coolant circuit, wherein the secondary coolant flows upward inside helical tubes in a counterflow arrangement with respect to the primary. coolant, and the ends of the helical tubes are attached to tube sheets which are accessible from above for inspection and repair, and a method for locating and repairing leaking conditions. The invention is applicable, for example, as an intermediate heat exchanger in a sodium to sodium system for large liquid metal fast breeder reactors.

6 Claims, 4 Drawing Figures PATENTED APR 2 3 I974 SHEET 1 BF 4 PATENTEDAPR 23 E974 SHEET 2 OF 4 BACKGROUND OF THE INVENTION The invention described herein was made in the course of, or under, Contract No. AT(04-3)-830 with the United States Atomic Energy Commission.

This invention relates to heat exchangers, particularly to an intermediate heat exchanger in a sodium cooled nuclear reactor, and more particularly to a heat exchange in which secondary fluid, such as sodium, flows inside helical tubes in counterflow relation with respect to the primary fluid, such as sodium, and wherein both ends of the helical tubes are attached to tube sheets which are accessible from above for inspection and repair.

Various types of heat exchangers utilizing helical coil are known in the prior art, as exemplified by U. S. Pat. Nos.: 1,839,133 issued Dec. 29, 1931; 1,922,149 issued Aug. 15, 1933; and 3,310,958 issued Mar. 28, 1967. However, a common problem exists in these prior known heat exchangers, that of removing the coil assembly from the containing vessel for repair, even minor repairs such as a leak in an individual tube. Heat exchangers, such as the intermediate heat exchangers, utilized in nuclear reactors, because of their great weight and size, as well as possible radioactive contami' nation on the shell side, make it very difficult and time consuming to remove same from the vessel for major or minor repairs or replacement of components, particularly when the most frequent mode of failure is le'a kage of the individual tubes.

SUMMARY OF THE INVENTION The present invention is directed to a heat exchanger utilizing helical coils constructed in a manner such that a leaking tube can be located and repaired or plugged without removal from the heat exchanger. This basically is accomplished by a helical tube construction wherein both ends of the tubes are attached to tube sheets which are accessible from above for inspection and repair. 7

Therefore, it is an object of this invention to provide a helical coil heat exchanger.

A further object of the invention is to provide a helical coil sodium to sodium heat exchanger.

Another object of the invention is to provide a helical heat exchanger wherein both ends of the coils are accessible for inspection and repair.

Another object of the invention is to provide a sodium-to-sodium heat exchanger wherein the secondary sodium flows inside helical tubes, and both ends of the tubes are attached to tube sheetswhich are accessible from above for inspection and repair.

Other objects of the invention will become readily apparent from the following description and accompanying drawings. I I

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a view, partially in cross section, of an embodiment of the inventive heat exchanger with portions cut-away for clarity;

FIG. 2 is an enlarged plan view, with portions thereof removed, of the FIG. 1 embodiment;

FIG. 3 is a view, partially in cross section, of another embodiment of .the invention, with sections omitted for clarity; and

FIG. 4 is a partial cross sectional view of another embodiment of the invention.

DESCRIPTION OF THE INVENTION The present invention involves a helical tube construction for a heat exchanger, such as used to transfer heat from a reactor-heated primary sodium coolant circuit to a secondary sodium circuit. The secondary sodium flows through the inside of the tubes in an upward helical pattern, while the primary sodium flows in countertlow downward over the outside surfaces of the helical tubes. The principal area of novelty resides in the number of tubes used and that both ends of the helical tubes are attached to tube sheets which are directly accessible for repairs through access ports. The invented heat exchanger can be used, for example, in a reactor system in which the reactor, primary pumps, and intermediate heat exchangers of the primary sodium circuit are contained within a single large tank or vessel, or in a loop type of system in which the reactor circuit is in a separate vessel connected by piping to the heat exchanger.

By way of example, the embodiments of the invention illustrated in FIGS. 1 to 3 are designed for use in a 350 MWe liquid metal fast breeder reactor (LMFBR), while the FIG. 4 embodiment is considerably larger and could be utilized in a 3,000 MWe LMF BR.

The embodiment of the invention illustrated in FIGS. land 2 comprises a helical U-tube heat exchanger indicated generally at 10 positioned in a primary tank 11 of a nuclear reactor and supported on the primary tank head 12, with sealing to the shield deck structure 13 provided by seal bellows 14, the primary tank 10containing liquid sodium (Na) coolant 15 having an upper level indicated at 15, which is hereafter described as the primary sodium. Heat exchanger 10 comprises one secondary sodium inlet tube section and three outlet tube sections (see FIG. 2) indicated generally at 16 and 17, inlet tube section 16 being connected to a secondary sodium inlet duct 18, while outlet tube sections are connected to a common plenum 19 which is connected to a secondary sodium outlet duct 20. Inlet tube section 16 and outlet tube sections 17 (only one shown in FIG. 1) are enclosed within a casing or inner shroud 21 open atthe lower end thereof which is mounted in an outer shroud or housing 22, also open at the lower end. Inner shroud 21 is provided in the central portion thereof with a plurality of openings or apertures 23 through which primary coolant from an inlet duct 24, connected to outer shroud 22, passes into' the outlet tube sections 17 and discharges therefrom at the open lower end 25 of inner shroud 22. A sleeve 26 is positioned around primary inlet duct 24 and the adjacent portion of outer shroud 22 providing an insulator space 27 therebetween Note that the'outer shroud 22 is enlarged adjacent apertures 23 so as to provide an annulus 28 whereby the primary coolant can readily flow around the tubes. of each of the three outlet tube sections 17. Heat exchanger 10 is provided with a top or cover 29 (see FIG. 2) through which tube sections 16 and 17 extend. Inlet tube section 16 comprises a casing 30 closed at the upper end with a removable cover or cap 31 to provide an access port to an inlet tube sheet 32 to which each of tubes 33 are secured at one end thereof (see FIG. 2). While only a'few tubes 33 are shown an actual heat exchanger may utilize about 2 inch O.D. Tubes, for example, each of which terminate in inlet tube sheet 32. Tubes 33 extend downwardly through casing 30 forming an inactive region 34 therein, outwardly through the open end 35 thereof into outlet tube sections 16, and upwardly around casing 30, the tubes 33 within inlet section 16 being equally divided into the three outlet sections 17. A gasfilled annular space 36 is defined between casing 30 and a shell 37 spaced thereabout (see FIG. 2); the casing 30 and shell 37 being joined by welding at their lower extremities 38, while the upper end of annular space 36 is exposed to the atmosphere of the room above the heat exchanger as more clearly seen in the FIG. 3 embodiment. Outlet tube sections 17 each comprise a casing 40 attached to inner shroud 21 at the upper portion of the shroud, and is open at the lower end and closed at the upper end by a removable cap or cover 41 forming an access port, tubes 33 extending upwardly around casing 30 and through casings 40 in a helical configuration as indicated as 33 passing through a stand pipe ring 56, and terminating in an outlet tube sheet 42 (only one shown in FIG. 1) having tubes 33' passing therethrough, thus both ends of the tubes are exposed and readily available for repair by removal of end caps 31 and 41. Stand pipe rings 56 are attached to stand pipes 57 which function to reduce thermal stresses during transients as the tube sheets will change temperature much more rapidly thanthe surrounding structure.

The operation of the embodiment of the inventive heat exchanger illustrated in FIGS. 1 and, 2 is as follows: primary sodium 15 enters the heat exchanger from the reactor vessel or tank 11 through the primary inlet 24, as indicated by the arrow 43 and passes via annulus 28 through apertures or holes 23 in the inner shell or shroud '21 into the interior of shellsor casings 40 of outlet tube sections 17, as indicated by the arrows 44. The sodium then passes downward across the exterior surface of the array of helical tubes 33' and discharges from the lower end 25 of the heat exchanger, as indicated by the arrows 45, whereafter it flows back into the main body 15 of sodium in the primary tank 11. Secondary sodium enters inletv tube section 16 through inlet 18, as indicated by arrow 46, and flows downwardly in casing 30, as indicated by arrows 47, through inlet tube sheet 32 into the tubes 33 which pass downwardly in inactive region 34 and into the helical coiled portions of the tubes indicated at 33' in outlet tube sections 17, wherein the secondary sodium is di rected upwardly through the helical coils which are in heat exchange relationship with the primaryvthrough outlet tube sheets 42 into outlet plenum 19, as 'indi cated by arrows 48,'and'out through outlet 20 to a point of use, as indicated by arrow 49.

It is thus seen that by removal of end caps or. covers 31 and 41 that the inlet tube sheet 32 and outlet tube sheets 42 are readily accessible for repair or plugging of defective tube 33-33 as discussed in greater detail hereinafter.

The embodiment of the invention illustrated in FIG. 3 is generally similar to that of FIG. 1 with the primary difference being that the helical tubes are in an elevated position suchthat the primary sodium must flow upwardly, compared to the FIG; 1 embodiment, before entering the holes in the shroud.=Accordingly, like componentswill be given similar reference numerals.

As pointed out directly above, the major difference be-' tween the FIG. 3 embodiment and that of FIG. 1 is in the location of openings or slots 23' in inner shroud 21, thus requiring the annulus 28' to be extended upwardly to direct primary coolant into slots 23. The helically coiled tubes 33 also extending further upward around casing and in casings of inlet tube sections 16, whereby there is a greater surface area thereof exposed to the downward flowing primary sodium. The sleeve 26 and insulative space 27' are also extended upwardly to surround the annulus 28. In addition, the secondary sodium discharges directly through individual outlets 20' and not via a common plenum asin FIG.

The operation of the FIG. 3 embodiment is similar to that'of FIG. 1 and is believed readily clear from the flow arrows without further description thereof.

The FIG. 4 embodiment, although substantially larger, is generally similar to FIGS. 1 and 3, with the main differences being in the positioning of the inlet tube sheet and the annular insulation space about the inlet tube section. Thus like components will be given similar reference numerals. Also, this embodiment utilizes four outlet tube sections instead of three as in'the other embodiments. More specifically, the inlet tube sheet 32 is located at a lower elevation in the casing 30 of inlet tube section 16 in order to minimize the pressure drop of the inactive region 34' of the tube 50,-

and to decrease the amount of tubing required. A slightly more complicated procedure isne'cessary with this tube sheet'po sitioning for inspecting and repairing 1 leaky tubes, andthis is discussed later in the description of the basic repair procedure. InFlG. 4, the annular gap insulation space 36 is sealed at the upper end, and a free or open surface 51 is maintained at a level near the bottom of the inactive region or section 34;0f tubing 50. This level may be either above or below inlet tube sheet 32', but should be sufficiently low to provide gas insulation where a large temperature differential is present. Gas surface 51 is essentially stagnant, and would be provided with a means of filling and controlling the level.

Because of the-very large size of the heat exchanger of FIG. 4, the inlet tube sheet 32' is made with penetrations that are smaller than the diameter of the tubes 50, permitting the tubing i'nthe inactive region 34 to be tightly packed ata centerline spacing essentially equal to the outside diameter of the tubes. At a level below the tube sheets 32 and 42 the tubes 50-50 are necked down to a smaller diameter, indicated at 52, before being welded to the tube sheet. It should be noted that I this feature is important to avoid a considerably larger overall heat exchanger diameter. r

In FIG. 4, the elevated heat exchanger concept, similar to FIG. 3 is used with primary sodium heat inlet 24 passing upward through annulus 28 before entering the openings or holes 23 in inner shroud 21'. Outlet tube sections 16are provided with thermal sleeves 53 and 54 which accept the temperature gradients and the mechanical pipe reactions. Also, theaccessports in FIG. 4, provided by removal of end caps 31 and 41',

are considerably smallerthan tube sheets 32' and 42,

compared to the FIGS. 1 and 3 emb odiments. These smaller access ports are compatible'with the illustrated method of support from thermal sleeves 53 and 54, since this method prevents straight access'to the outer edges of the tube sheets 32 and 42 A support 55 between the heat exchanger 10 and the shield deck structure 13 is shown only in a simplified representation.

The FIG. 4 embodiment may be modified to position the tube sheets 32' and 42' on the same elevation as in the FIGS. 1 and 3 embodiments. Also, a cover gas, such as argon, may be used in the region above the primary inlet region.

The heat exchanger units or 10 can be removed from the vessel or tank 1 1 for major repairs or replacement. Their great size and weight however make this a very difficult and time consuming operation. Since the leakage of individual tubes is the most likely mode of failure, a method has been provided for locating such a leak and plugging individual tubes without removing the heat exchanger. Such a repair can be made with only a short reactor shutdown time. The following procedure is based on the embodiment of FIG. 4, but could be used with modificationsfor the other embodiments.

Since the secondary sodium operates at a higher pressure than the primary sodium, a leak will cause secondary sodium to pass into the primary system. This becomes a problem only when the leak rate is higher than can be accommodated by the capacities of the primary and secondary sodium systems. Initial indication of such a leak would be a lowering of the sodium level in a secondary loop which is greater than can be accounted for by other phenomena such as thermal expansion, overflow into drain tanks, etc. When a repair becomes necessary, the reactor would be shut down and the primary system cooled to a lower temperature (400-600F). The following steps would then be taken:

' 1. Drain the secondary sodium until the level is near the tops of the outlet tube sheets 42', replacing the sodium with argon gas.

2. Remove a shield plug above the end cap 31' for the inlet tube sheet 32. It may be necessary to provide temporary shielding around this hole prior to removing the shield plug to protect personnel from radiation from the reactor and primary sodium. A slight inflow of argon into the space around the piping will minimize the entrance of air.

3. Open the access port of inlet tube section 16 by removing the end cap 31, using remote equipment.

4. Install combination periscope and tube plugging device onto access port. This device includes a flexible joint to permit it to be oriented in line with any of the tube sheet holes.

5. Continue draining the secondary sodium in the inlet tube section, while controlling differential argon pressure between the inlet and outlet sections to drain the sodium to the levels of both the inlet and outlet tube sheets. This requires that the.

inlet 18' be maintained at a higher gas pressure than the outlets 20' because of its lower inlet tube sheet position.

6. Using the periscope, examine-the inlet tube sheet 32'. Sodium can be seen in each of the tubes 50, unless the tube is leaky. A leaky tube will drain into the primary system, and its depressed level will be apparent. If no such leak can be detected, the argon pressure can be raised while maintaining the differential between the inlet and outlets) to check for high pressure leaks.

7. Plug the leaky tube with the tube plugging device, and carefully make note of its location in the tube sheet.

8. Remove the repair device, close the access port by replacing the end cap, and replace the shield plug.

9. Determine the location of the other end of the plugged tube, and repeat the plugging operation at the appropriate outlet tube sheet via the access port therefor. In this portion of the repair operation it is not necessary to maintain the differential pressure between the inlet and outlets.

During the above-described inspection and repair operations, no direct access is required to the primary sodium system, except through the leakage path. It should be possible to minimize contamination of the secondary sodium by maintaining the secondary pressure at a level higher than the primary pressure.

It has thus been shown that the present invention providesa heat exchanger wherein the secondary fluid flows in helical tubes in counterflow relation to the flow of the primary fluid, and wherein both ends of the tubes are attached to tube sheets which are accessible from above for inspection and repair.

While particular embodiments of the invention have been illustrated and described, modifications and changes will become apparent to those skilled in this art, and it is intended to cover in the appended claims all such modifications as come within'the spirit and scope of the invention.

What we claim is 1. A heat exchanger comprising: a centrally located inlet tube section; a plurality of outlet tube sections positioned around said central inlet tube section; said inlet tube section and said outlet tube sections being provided with a plurality of tubes each having a portion extending in one direction through said inlet tube section and another portion extending in the opposite direction through said outlet tube sections; said portion of said tubes extending through said outlet tube section being at least partially in a helical configuration; said inlet tube section and said outlet tube sections each being provided with a tube sheet means; each of said tubes terminating at one end thereof in said tube sheet means of said inlet tube section and at the opposite end thereof in said tube sheet means of one of said outlet tube sections; means for directing fluid into said inlet tube section, through said tubes, and discharging same from said outlet tube sections; means for directing a different fluid across the outer surface of at least said helical configured portion of said tubes in a counterflow direction to that of the fluid within said helical configured portion of said tubes; removable means provided in each of said inlet and outlet tube sections at a location directly above each of said tube sheet means thereof, whereby said tubes having the ends thereof in said tube sheet means are accessible from above for inspection and repair; said inlet tube section including a casing means positioned centrally within a shroud means, said shroud means having apertures in a side portion thereof and being open at one end; said outlet tube sections each including a casing means positioned in spaced relation within'said shroud means and with respect to said casing means of said inlet tube section; insulation means defined by a gas-filled space between at least the upper portion of said casing means 'of said inlet and outlet tube sections; said casing means of said inlet and outlet tube sections being open at one end thereof through which said plurality of tubes pass from said inlet tube section to said outlet tube sections; said means for directing the different fluid includes means defining an annulus positioned around at least said apertured side portion of said shroud means, and fluid inlet means connected to said annulus, whereby the different fluid passes through said fluid inlet means, said annulus, and said apertures into said outlet tube sections; and insulator means provided about said means defining said annulus.

2. The heat exchanger defined in claim 1, wherein said tube sheet means in each of said inlet and outlet tube sections are located at approximately the same elevation.

3. The heat exchanger defined in claim 1, wherein said tube sheet means in said inlet tube section is located at an elevation different from the elevation of said tube sheet means in each of said outlet tube tions.

4. The heat exchanger defined in claim 1, wherein said apertures are located in an upper side portion of said shroud means.

5. The heat exchanger defined in claim 1, wherein said apertures are located in approximately the central side portion of said shroud means.

6. Thee heat exchanger defined in claim 1, wherein said means for directing fluid into said inlet tube'section includes fluid inlet means intermediate said tube sheet means and said removable means, and wherein said means for discharging the fluid from said outlet tube sections includes a common plenum connected to each of said outlet tube sections and fluid outlet means connected to said plenum. 

1. A heat exchanger comprising: a centrally located inlet tube section; a plurality of outlet tube sections positioned around said central inlet tube section; said inlet tube section and said outlet tube sections being provided with a plurality of tubes each having a portion extending in one direction thRough said inlet tube section and another portion extending in the opposite direction through said outlet tube sections; said portion of said tubes extending through said outlet tube section being at least partially in a helical configuration; said inlet tube section and said outlet tube sections each being provided with a tube sheet means; each of said tubes terminating at one end thereof in said tube sheet means of said inlet tube section and at the opposite end thereof in said tube sheet means of one of said outlet tube sections; means for directing fluid into said inlet tube section, through said tubes, and discharging same from said outlet tube sections; means for directing a different fluid across the outer surface of at least said helical configured portion of said tubes in a counterflow direction to that of the fluid within said helical configured portion of said tubes; removable means provided in each of said inlet and outlet tube sections at a location directly above each of said tube sheet means thereof, whereby said tubes having the ends thereof in said tube sheet means are accessible from above for inspection and repair; said inlet tube section including a casing means positioned centrally within a shroud means, said shroud means having apertures in a side portion thereof and being open at one end; said outlet tube sections each including a casing means positioned in spaced relation within said shroud means and with respect to said casing means of said inlet tube section; insulation means defined by a gas-filled space between at least the upper portion of said casing means of said inlet and outlet tube sections; said casing means of said inlet and outlet tube sections being open at one end thereof through which said plurality of tubes pass from said inlet tube section to said outlet tube sections; said means for directing the different fluid includes means defining an annulus positioned around at least said apertured side portion of said shroud means, and fluid inlet means connected to said annulus, whereby the different fluid passes through said fluid inlet means, said annulus, and said apertures into said outlet tube sections; and insulator means provided about said means defining said annulus.
 2. The heat exchanger defined in claim 1, wherein said tube sheet means in each of said inlet and outlet tube sections are located at approximately the same elevation.
 3. The heat exchanger defined in claim 1, wherein said tube sheet means in said inlet tube section is located at an elevation different from the elevation of said tube sheet means in each of said outlet tube sections.
 4. The heat exchanger defined in claim 1, wherein said apertures are located in an upper side portion of said shroud means.
 5. The heat exchanger defined in claim 1, wherein said apertures are located in approximately the central side portion of said shroud means.
 6. Thee heat exchanger defined in claim 1, wherein said means for directing fluid into said inlet tube section includes fluid inlet means intermediate said tube sheet means and said removable means, and wherein said means for discharging the fluid from said outlet tube sections includes a common plenum connected to each of said outlet tube sections and fluid outlet means connected to said plenum. 